Amplifiers are used to increase the strength or amplitude of an electrical signal. One common type of amplifier is an operational-amplifier (op-amp), a high-gain voltage amplifier having a pair of differential inputs and an output. The op-amp amplifies the difference between the voltages at the inputs by a factor of hundreds or thousands and presents the amplified difference at the output. Op-amps may be connected in a number of circuit configurations to control the gain and to perform various functions.
When designing an op-amp, a trade-off exists between the amplifier's noise-to-power ratio and its slew rate. The noise-to-power ratio is the ratio of noise performance, noise generated by the op-amp, to power consumption, the power consumed by the op-amp. Transistors in the input differential stage op-amp are the main contributors to noise in the op-amp. To minimize noise, their transconductance (gm), the ratio of output current change to input voltage change, needs to be maximized, while keeping the bias current and thus overall power consumption as low as possible. For junction field-effect transistor (JFET) and metal-oxide-semiconductor field-effect transistor (MOSFET) input devices, this is done by ensuring that they operate in weak-inversion, where their gm/drain current (gm/Id) ratio is as high as possible. Because the amplifier needs to be stable, Miller compensation is typically applied, increasing the input capacitance and reducing the bandwidth of the op-amp. The required size of the Miller capacitor is mainly determined by the gm of the input stage, as the dominant pole is determined by gm/CMiller. Thus, a choice for a low noise design implies high gm, and thus requires a large Miller capacitor.
The slew rate of the op-amp, the maximum rate of change in the output voltage, is generally determined by the ratio of the tail current (2*Id) and the Miller capacitor. As outlined above, a power-efficient low-noise design leads to a large Miller capacitor and a relatively low drain current. Thus, such a low noise design will have a poor slew-rate for given tail current and power consumption. A need therefore remains for an amplifier having a power-efficient low-noise amplifier with a high slew rate.